392 VAPOR-DENSITIES. 



The quantity of acid diminishes somewhat regularly from '2084 

 grams in series A to '0185 in series K. The volume, which was 

 154 in the experiment at 185 in series A, diminishes in the 

 successive series, and in the same series with diminishing temperature, 

 to 69'6 OC in the experiment at 78 in series K. It is worthy of notice 

 that the greatest deviations from the formula occur where the liability 

 to error is most serious with respect to pressure (which was measured 

 without a cathetometer), to volume, and to the quantity of acid. 



Far more serious than the absolute amount of these divergences, is 

 the regularity which they exhibit. But it must be remembered that 

 the observations are by no means entirely independent, and many 

 sources of possible error, such as the calibration of the tube and the 

 determination of the quantity of acid, might affect the results with 

 considerable regularity. 



Only to a slight degree can the divergences from the formula be 

 accounted for by an insufficient exposure to the temperature of the 

 experiment. The observations, except those at 78, were made with 

 increasing temperatures, and the greatest divergences from the formula 

 are not in the positive direction. Yet the positive divergences occur 

 where we should most expect to find them, if they were due to this 

 cause, viz., in the series in which the greatest quantities of acid were 

 used, and in cases in which the temperature seems to have been 

 raised at once an unusual number of degrees. (See especially the 

 observation at 120 in series D, 'and in general the observations at 

 185, which exhibit if not a positive at least a diminution of negative 

 excess.) In the observations at 78, which were the last of each 

 series, and therefore followed a fall of temperature from 185, we find 

 in some cases, especially in series G, H, and J, a negative divergence 

 much greater than in the other determinations of the same series, and 

 which appears to be referable to this circumstance. 



In Table VI are exhibited the results of experiments by Playfair 

 and Wanklyn,* in which the vapor of the acid was diluted with 

 hydrogen or, in a single case (the experiment at 95'5), by air. 

 Columns I and II of the observed densities relate each to a series of 

 observations by the method of Gay-Lussac, column III contains four 

 independent determinations by the method of Dumas. The numbers 

 in the column of pressures are, as in other similar cases, the partial 

 pressures obtained by subtracting from the total pressure (which was 

 never very much less than that of the atmosphere) that which would 

 be exerted by the hydrogen or air alone. 



The first observation of the first series gives the density T936, 

 which is doubtless too small, since it is much less than the theoretical 



* Trans. Roy. Soc. Edirib., vol. xxii, p. 455. 



